It is known from U.S. Pat. No. 2,395,800 that 1,3-diketones can be prepared by thermal rearrangement of the corresponding isomeric enol esters at a temperature of from 300.degree. C. to 700.degree. C. in a tube reactor, with the gaseous product mixture subsequently being condensed and the pure product being distilled off. A disadvantage is that free-radical mechanisms leading to undesirable by-products and to carbon formation proceed in reactions of this type and thus make it necessary to stop production for cleaning the reactor. These by-products reduce the selectivity and have to be separated off by distillation. DE-A2047320 (U.S. Pat. No. 3,794,686) describes a procedure in which a tetraalkyllead is added to the reaction mixture to prevent carbon deposition. The Derwent Abstract--JP-A63159337--describes the addition of water to improve the selectivity in the preparation of acetylacetone from isopropenyl acetate and at the same time, reduces the rate of carbon formation. However, a disadvantage is that the water thus introduced leads firstly to hydrolysis of the enol ester and secondly, to corrosion problems.
It is therefore an object of the invention to develop a process which gives a higher conversion at least equal selectivity and lower carbon formation without addition of promoters of water.